[ Build and deploy an effective line of defense against corporate intruders with InfoWorld's Encryption Deep Dive PDF expert guide. Download it today! | Stay up to date on the latest security developments with InfoWorld's Security newsletter. ]

Trend found a "backdoor," or a semi-secret way to access the device, Yeh wrote. The password needed to open up the backdoor is hardcoded into the device's firmware. All of the routers appear to have the same password.

"Attackers can easily log into these routers, and users cannot modify or disable this backdoor," he wrote.

Backdoors can be used for legitimate product support, but coding such access methods into software is generally discouraged for fear of abuse.

The Netcore and Netis routers have an open UDP port, 53413, which can be queried since the routers have an externally accessible IP address, Yeh wrote. Trend Micro scanned the Internet and found more than 2 million IP addresses with the open UDP port, which could indicate vulnerable equipment.

"Almost all of these routers are in China, with much smaller numbers in other countries, including but not limited to South Korea, Taiwan, Israel, and the United States," Yeh wrote.

Trend Micro has notified the company but did not receive a response. Company officials reached in Shenzhen didn't have an immediate comment.

By using the backdoor, an attacker could upload or download files to the device. A router's settings could also be changed to allow a hacker to monitor a person's Internet traffic as part of a man-in-the-middle attack, Yeh wrote.

Trend also found that a file containing a username and password for the routers' Web-based administration control panel is stored unencrypted, which could be downloaded by an attacker.

It doesn't appear that most Netcore and Netis routers support the installation of other open-source firmware packages, such as dd-wrt or Tomato, that could be used to replace the vulnerable software, Yeh wrote.

"Aside from that, the only adequate alternative would be to replace these devices," he wrote.